The Webb Space Telescope Will Rewrite Cosmic History. If It Works.

Kepler delivered on Earth-size planets. Kepler 10b was identified in the first 10 days of data we got back from the spacecraft, Batalha said. When they graphed the brightness of the host star over time, the dip was visible to the eye. Follow-up observations from the ground confirmed it was a genuine planet and one that, based on its mass and radius, had to be rocky. Batalha presented the unequivocal detection in January 2011, following a more tentative claim of a rocky exoplanet labeled CoRoT-7b by astronomers in Europe. Neither Kepler 10b nor CoRoT-7b earned the coveted designation Earth-like, because they orbited near their parent star rather than in the habitable zone, where water is liquid. (The first rocky, watery and potentially Earth-like planet, Kepler 186f, made headlines in 2014. Batalha wasnt formally involved with the analysis.)

The Kepler telescope, before being prematurely hobbled by the failure of two of its motors, discovered more than 2,600 exoplanets. More than 4,500 have been counted in all, a sufficient number for astronomers to study their statistical properties. Just as 51 Pegasi b had suggested, our solar system is atypical. For instance, the most common type of planet in the galaxy is a size we dont have, in between rocky planets and giants. Planetary astronomers dont yet understand the surplus of these so-called super-Earths or sub-Neptunes, or what these midsize planets are like, or how they form. New principles of planet formation and evolution are needed.

Extrapolating the data so far, researchers think that our galaxy holds billions of rocky, watery planets, suggesting that life, too, might be common. Until we find evidence of life actually inhabiting another planet, though, it remains possible that its emergence on Earth was a fluke, and that we are alone.

Happily, the Webb telescope will be powerful enough to probe the atmospheres and climates of other Earths or even, if were very lucky, find evidence of an actual alien biosphere.

Infrared is fantastic for exoplanets, Batalha said.

One Strike and Youre Out

One morning in 1987, the astrophysicist Riccardo Giacconi, who was then the director of the Space Telescope Science Institute (STScI) and of the yet-to-launch Hubble, asked deputy director Garth Illingworth to start thinking about Hubbles successor. My immediate reaction is, Argh, we havent even got Hubble launched yet, and weve got a million things to do on there it has major problems so how can we do this as well? Illingworth recalled recently. He said, Trust me, youve got to start early because I know it takes ages to do this. Hubble had been under development since around 1970, spearheaded in its early years by the NASA astronomer Nancy Roman following decades of campaigning by Princetons Lyman Spitzer; they are known as the mother and father of Hubble.

Illingworth, who is from Australia, got together with his STScI colleagues Pierre Bely of France and Peter Stockman of the U.S. to brainstorm about the next-generation space telescope. They had basically nothing to go on. We started thinking about what would be good to go beyond Hubble and to complement whatever it did and explore new areas, Illingworth said, and the IR was one clear area. Infrared light is prohibitively difficult to observe from the ground. The trio figured that in space, where the infrared background is more than 1 million times lower, there would be plenty to see. When you put a powerful new capability out there, you open an immense number of scientific horizons.

For an IR telescope to be as sensitive as Hubble, which has a 2.4-meter-wide primary mirror, Illingworth, Bely and Stockman realized that it would need to be significantly bigger, since it detects bigger wavelengths. They considered that the mirror might have to fold to fit in a rocket. They also knew it had to be cold, otherwise its heat would saturate its own sensors. Rather than actively cool the telescope, they thought to exploit the extreme frigidity of outer space by blocking the heat of the Earth, moon and sun. Their vague conception of a large, passively cooled infrared telescope, greatly elaborated upon, would become the cargo now awaiting launch in Kourou.

Leading astronomers convened at STScI in 1989 to discuss the science that an infrared space telescope might be good for. Discussions slowed during Hubbles disastrous start and salvation, then picked up again in the mid-90s. In 1995, John Mather, a reedy, gentlemanly astrophysicist at the Goddard Space Flight Center, got a call from NASA headquarters asking if hed like to join the project. Realizing that an infrared telescope would do so much for so many people, he dropped everything and signed on. Hes been JWSTs top scientist ever since.

Mather calls himself a theoretical instrument builder. He started building telescopes as a kid in pastoral New Jersey, assembling parts from catalogs in the hope of getting a closer look at the surface of Mars. As a young man in the 1970s, Mather worked on a balloon-borne instrument that failed; he and his colleagues concluded that they hadnt tested it enough before launch. Murphys law had been proven one more time, he wrote in an autobiographical account. But lessons learned led to the triumph of COBE, the NASA satellite experiment for which he and George Smoot would share the Nobel. In the early 90s, COBE measured the subtle variations in the cosmic microwave background that are thought to have seeded all later cosmological structures. In Mathers mind, theorizing about the cosmos is fine, but you need ingenious instruments to know anything for sure. So lets build the equipment, he told me this fall. To me thats a heroic thing to do.